Pitch control systems having blade pitch angle adjusting apparatuses are known from the state of the art.
In these systems, each rotor blade is pitched individually with a dedicated blade pitch angle adjusting apparatus, namely an electromechanical actuator. Each such actuator comprises an electric pitch motor, a reducer and an electromagnetic pitch brake.
By default, the pitch brake is in a braking condition. In order to adjust the pitch angle, the pitch brake is released, and the pitch motor drives the blade in a rotary motion around its longitudinal axis. When reaching the desired pitch angle, the pitch brake returns to its default braking condition and stops the rotating blade.
Such a blade pitch angle adjusting apparatus has the drawback that the pitching of the blade is no longer possible when the pitch brake fails. When such a failure occurs, the pitch brake cannot be released and consequently the pitch motor operates against a closed brake. This can cause an overload to the motor's electric circuit and damage the motor. In any case, the pitch motor is unable to turn the blade. Accordingly, the wind turbine looses its pitching capability, can no longer adapt to varying wind conditions, and risks to be subjected to dangerously high loads caused by aerodynamic imbalances.
A solution to this problem is to provide the blade pitch angle adjusting apparatus with a high power pitch motor that is able to overcome the braking force of the pitch brake in case of brake failure. However, this solution is not satisfying since it increases the size, cost and weight of the pitch motor.
Document RU 2 354 845 C1 discloses a blade pitch angle adjusting apparatus which, instead of having an electric pitch motor, exploits the kinetic energy of the wind turbine for pitching.